A speed reducer having a planetary gear mechanism is used for reducing the speed of rotation of a motor, for example. As one type of the planetary gear mechanism, there is widely known a planetary gear mechanism having a first shaft, an externally toothed gear mounted on the first shaft via an eccentric body provided on the first shaft so as to be eccentrically rotatable relative to the first shaft, an internally toothed gear which the externally toothed gear internally contacts and meshes with, and a second shaft coupled to the externally toothed gear via a means for transmitting only the rotational component of the externally toothed gear. As a specific example of this type of planetary gear mechanism, a cycloid differential planetary gear mechanism is known.
The speed reducer using the cycloid differential planetary gear mechanism achieves a large speed reduction ratio with one reduction stage, and operates with a high efficiency owing to a high contact ratio, as compared with a planetary gear mechanism having general gears of an involute tooth profile. On the other hand, the speed reducer of the cycloid type has a complicated mechanism for taking output out of eccentric oscillating rotation, and is likely to be available at a high cost.
A general cycloid differential planetary gear mechanism causes an epitrochoid externally toothed gear to internally contact a pin gear as an internal gear and eccentrically oscillate, so as to provide output via inner pins. In order to cancel out an imbalance due to the eccentric arrangement, an externally toothed gear having the same structure as and opposite in phase to the above-indicated externally toothed gear is added, or two pieces of externally toothed gears having the same structure and shifted in phase by 120° from each other in terms of the direction of displacement of the center are added (see Patent Document 1).
However, the known cycloid differential planetary gear mechanism makes it necessary to machine grooves in which pins are to be embedded, with high accuracy. so as to provide a circular arc inner gear formed by the pins. Furthermore, the pins need to be provided over the entire circumference of the inner gear, and the highly accurate machining needs to be done for the number of the pins.
Also, the epitrochoid gear of the externally toothed gear that externally contacts the pins is required to be machined with high accuracy.
Furthermore, the inner pins that extend through the externally toothed gear need to be provided, and high accuracy is also required for the positions and size of holes through which the inner pins extend.
Thus, high machining or working accuracy is required for each of components that constitute the known cycloid differential planetary gear mechanism, resulting in a problem of increase in cost.
Furthermore, the overall construction of the known cycloid differential planetary gear mechanism is complicated, and the inner pins are arranged to extend through the externally toothed gear; therefore, there is a limitation to reduction of the outside diameter thereof.